1. Field of the Invention
The present invention relates to a method of wire bonding and to a semiconductor device manufactured through the method. More particularly, the present invention relates to a method of wire bonding enabling high speed bonding and applicable to thin packages, compact-sized, large capacity packages and multifunctional packages, as well as to a semiconductor device manufactured through the use of the method.
2. Description of the Background Art
In order to meet the current trend of semiconductor package developments of compact size, large capacity and multiple functions, there have been various requirements on the method of wire connection between a semiconductor element in a package and an external terminal of the package.
A reverse loop such as shown in FIG. 8, or a chip-to-chip loop such as shown in FIG. 9 has been used for a thin package, a compact-sized, large capacity package and a multifunctional package.
When a reverse loop such as shown in FIG. 8 is utilized, an inner lead 10 and a bonding pad 6 are connected by means of a first ball 2, a bonding wire 1, and a stud bump (second ball) 9. Bonding pad 6 is formed on a semiconductor device (chip) 7 mounted on a die pad 8.
When a chip-to-chip loop such as shown in FIG. 9 is utilized, bonding pads 6 on semiconductor device 7 are connected to each other by means of first ball 2, the bonding wire 1, and stud bump (second ball) 9.
In the reverse loop or chip-to-chip technique, secondary bonding is performed on the bonding pad 6 on the chip. Here, a stud bump 9 is formed in advance on bonding pad 6 as shown in FIG. 10, and the secondary bonding is performed on stud bump 9, using a capillary 4 and a wire cut damper 5 as shown in FIG. 11. Namely, an on-bump secondary bonding technique is used. In the on-bump secondary bonding technique, the step of arranging a stud bump is necessary, separate from the step of arranging the wire. This results in larger number of steps required for wire bonding, resulting in low efficiency in manufacturing the semiconductor devices.
There is a wire bonding technique that overcomes the disadvantages of the on-bump secondary bonding technique and improves productivity. In this technique, ball bonding is performed both for the primary and secondary bondings (hereinafter referred to as xe2x80x9cball-to-ball bondingxe2x80x9d). FIG. 12 shows an example in which inner lead 10 and a bonding pad 6 are connected by means of bonding wire 1 through ball-to-ball bonding.
In the ball-to-ball bonding, primary bonding is performed by forming a first ball 2 by common spark discharge at a wire tip portion. Thereafter, spark discharge is performed using a torch electrode 11 at a desired wire ball forming portion as shown in FIG. 13, so that a melted ball (second ball) 13 is formed as shown in FIG. 14. Thereafter, air is blown out from an air outlet 12 to hold the melted ball 13. The melted ball 13 is bonded to the bonding pad 6 as shown in FIG. 15, and thus the secondary bonding is performed.
When the second ball is formed by melting the bonding wire in the above described manner, however, it is difficult to form the second ball at a prescribed position with high accuracy. Therefore, loop control becomes difficult, as the wire becomes too tense or too slack for the wire loop.
As stable formation of the second ball by melting is difficult, joining attained by the second ball bond is unstable.
Accordingly, stable and high-speed ball-to-ball bonding is difficult, which degrades productivity of the semiconductor device and, in addition, poses questions on the quality of the semiconductor devices.
The present invention was made to solve the above described problems. An object of the present invention is to improve not only the quality but also the productivity of the semiconductor devices, by performing stable and high-speed ball-to-ball bonding.
According to the present invention, the semiconductor device includes a first conductive layer, a first ball (bump) formed on the first conductive layer, a second conductive layer positioned spaced apart from the first conductive layer, a second ball (bump) formed on the second conductive layer, and a bonding wire connecting the first and second balls. The second ball is formed by mechanically deforming the bonding wire. Here, xe2x80x9cballxe2x80x9d refers to a lump conductive portion, including conductive portions having spherical shapes and other shapes.
As the second ball is formed by mechanically deforming the bonding wire as mentioned above, it is possible to form the second ball by mechanically deforming the bonding wire after the bonding wire is preliminarily joined to the second conductive layer. More specifically, it is possible to perform the secondary bonding after the shape of the wire loop is established. This enables easy and highly accurate loop control of the bonding wire. Further, as the second ball is formed by mechanically deforming the bonding wire, it is possible to form the second ball in a more stable and easier manner, as compared with formation of the melted ball.
The second ball may be formed by bending the bonding wire on the second conductive layer, or the second ball may be formed by making the bonding wire curved on the second conductive layer. In either way, the bonding wire can be mechanically deformed to attain the above described effects.
The first conductive layer includes an inner lead, and the second conductive layer includes a bonding pad. As the present invention is applied to connection between the inner lead and the bonding pad, it becomes possible to improve controllability of the loop shape of the bonding wire connecting the inner lead and the bonding pad and, in addition, it becomes possible to ensure reliability and stability of joint on the bonding pad.
The semiconductor device may include a base, a semiconductor device mounted on the base with a die pad interposed, a sealing resin sealing the semiconductor device, and an external terminal formed on a rear surface of the base. Here, the first conductive layer includes a land formed on the base, and the second conductive layer includes a bonding pad formed on the semiconductor device.
The present invention is also applicable to connection between the land formed on the base and the bonding pad formed on the semiconductor device by means of a wire. In this case also, controllability of the loop shape of the bonding wire is improved while stability and reliability of joint on the bonding pad can be ensured.
The semiconductor device may include a base, first and second semiconductor devices mounted on the base with a die pad interposed, a sealing resin sealing the first and second semiconductor devices, and an external terminal formed on the rear surface of the base. Here, the first conductive layer includes a first bonding pad formed on the first semiconductor device, and the second conductive layer includes a second bonding pad formed on the second semiconductor device.
The present invention is also applicable to wire connection between bonding pads formed on the first and second semiconductor devices mounted on the base. In this case also, controllability of the wire loop shape is improved while stability and reliability on the bonding pads can be ensured.
According to the present invention, the method of manufacturing a semiconductor device includes the following steps. First, a first ball formed at the tip of the bonding wire is joined to a first conductive layer (first bonding step). After the first bonding step, the bonding wire is joined to the second conductive layer. With the bonding wire joined to the second conductive layer, the bonding wire is mechanically deformed on the second conductive layer. The deformed portion of the bonding wire is joined to the second conductive layer (second bonding step).
As the bonding wire is preliminarily joined to the second conductive layer after the first bonding step, the shape of the wire loop can be established. Therefore, as compared with the prior art in which the melted ball is formed, control of the wire loop is easier. Further, as the bonding wire is mechanically deformed with the bonding wire joined to the second conductive layer, the deformed portion of the bonding wire can surely be formed at a desired position. As the second ball can be formed simply by joining the deformed portion to the second conductive layer, the second ball can be formed easily and stably. Further, as the deformed portion of the bonding wire is joined to the second conductive layer after the bonding wire is once joined to the second conductive layer, stability of the joint between the second ball and the second conductive layer can be improved.
The step of mechanically deforming the bonding wire may include the step of bending the bonding wire on the second conductive layer, or it may include the step of making the bonding wire curved on the second conductive layer. By either step, the bonding wire can be deformed mechanically on the second conductive layer, and hence the above described effects can be attained.
The bonding wire is held by a bonding tool such as a capillary. Here, the step of mechanically deforming the bonding wire includes the step of mechanically deforming the bonding wire on the second conductive layer by moving the bonding tool with the bonding wire being joined to the second conductive layer.
As the bonding tool is moved with the bonding wire joined to the second conductive layer, it becomes possible to feed the bonding wire from the bonding tool on the second conductive layer, and the fed portion can be mechanically deformed on the second conductive layer.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.